The work is divided into sections, the first of which consists of a brief historical _resumé_, the opinions of Acosta, Alpinus, Ray and Linnaeus on this subject being alluded to. No mention, however, is made of the observations of Bonnet and of Mairan to the effect that the periodic movements of _Mimosa pudica_ continued when the plant was kept in prolonged darkness. In Section 2, after describing the structure of a leaf, Hill remarks that "Leaves are always surrounded by the air; and they are occasionally and variously influenced by heat, light, and moisture. They are naturally complicated, and they act on most occasions together. We are therefore to observe, first, what effects result from their mutual combinations in a state of nature: and having assigned in these cases the effect to the proper and particular cause, from this power of that agent, whichsoever it is, that acts thus in concert with the rest, we may deduce its operations singly." This passage, although not particularly clear, indicates that Hill fully appreciated the fact that the reaction exhibited by a plant organ is a response to the resultant of a number of forces, and that each factor must be examined separately. He then goes on to describe his observations on _Abrus_; the structure of the leaf, more especially the course of the vascular bundles, is first dealt with, and then an explanation of the action of light is given. Needless to say, in view of the state of physical science at this period, his explanation, although ingenious, is wide of the mark. He wrote that "Light is subtile, active, and penetrating: by the smallness of its constituent parts, it is capable of entering bodies; and by the violence of its motion, of producing great effects and changes in them. These are not permanent, because those rays which occasion them, are, in that very action, extinguished and lost. "Bodies may act on light without contact; for the rays may become reflected when they come extreamly near: but light can act on bodies only by contact; and in that contact the rays are lost. The change produced in the position of the leaves of plants by light, is the result of a motion occasioned by its rays among their fibres: to excite this motion, the light must touch those fibres; and where light touches, it adheres, and becomes immediately extinguished.... The raising of the lobes in these leaves will be owing to the power of those rays which at any one instance fall upon them: these become extinguished; but others immediately succeed to them, so long as the air in which the plants stands, is enlightened."